A conventional metal airframe construction provides a good Faraday cage that shields internal electronic equipment from electromagnetic fields. However the increasing use of composite materials in airframe construction has left internal electronic equipment vulnerable to electromagnetic fields and has necessitated either the provision of heavy protective sheathing for such electronic equipment in the vicinity of composite airframe sections, or the replacement of such vulnerable electronic equipment with equipment which is not susceptible to electromagnetic fields. The latter alternative has lead to the adoption of optical encoders for sensing relative linear or rotary positions, for instance, for monitoring the position of control surfaces. In addition to being immune to electromagnetic fields, the optical fibres require minimal protective sheathing and are much lighter than heavily sheathed electrical wiring.
U.S. Pat. No. 3,514,617 is concerned with a system for reading utility meters from a central location by means of a telephone circuit and particularly to an improved optical transducer for providing a signal indicative of a meter reading. More specifically it teaches that a utility meter pointer shaft should drive a plate having a reflective surface on which is drawn a pattern of nonreflective radial line segments that start from a reference angular position and have a continuously increasing angular spacing extending along a circumferential path. A cantilever is oscillated by the action of two electromagnets and carries two light pipes. The pattern is illuminated from a stationary light source through one of the light pipes, the vibration of the cantilever causing the free end of this light pipe to scan the transmitted light over a portion of the path on the reflective disc surface including a large number of the closely spaced radial line segments. A stationary photoresponsive device senses light reflected from the relative portions of the disc through the other light pipe. In this way a square wave signal is developed at an output of the photoresponsive device and has a fundamental frequency dependent upon the spacing of the nonreflective radial line segments and the vibrational speed of the cantilever. The fundamental frequency signal is then filtered out to provide a signal indicative of the position of the meter pointer shaft. Although this teaching enables the position of the utility meter pointer shaft to be sensed from a remote position, it inherently relies on a vibrating cantilever to scan the light over the pattern, and electromagnets to vibrate the cantilever. These features are clearly unsuitable for use in vehicles as the movement of the cantilever would be modulated by any sudden movement of the vehicle and would cause a false reading. In the case of an aircraft subject to sudden high loading, such modulation of the cantilever movement would cause a false reading at the very moment when an accurate indication is required, for instance, of one or more of the control surfaces. Furthermore there is also the danger that the cantilever vibration may be affected by the transmission of vibrations generated elsewhere in the airframe. The electromagnets vibrating the cantilever would cause an electromagnetic field within the airframe which, as already stated, is unacceptable.
Although the present invention is primarily concerned with optical encoders for use in avionics applications, such optical encoders are also suitable for other applications susceptible to electromagnetic radiation or where electrical sparks could be hazardous, and indeed could be used on machine tools or in any application requiring the sensing or measurement of the relative position of two members.
Many optical fibre sensor techniques have been demonstrated capable of measuring almost all measurands. Most of such techniques, especially for the measurement of linear and rotary position, are intensity based--that is the optical power is a function of the measurand. When any optical component of such an intensity based sensor system is replaced (e.g. the sensor head, the fibre link, or the electro-optics unit), recalibration of the system is necessary due to the variability of component and connector losses. The optical losses at each connector vary widely from as little as 0.1 decibel to as much as 2 decibels for a poor connection. It is for this reason that optical intensity based fibre optic sensors have not yet become a viable alternative to electrical sensors. Indeed, for applications in the avionics field, the cost and time delay required for such recalibration is unacceptable.